From American document U.S. Pat. No. 5,137,228, it is already known a rotary wing blade capable of being torsion-deformed about an axis at least approximately along the direction of the span of said blade and comprising, connected to each other, a front longitudinal portion comprising the leading edge and a rear longitudinal portion comprising the trailing edge. In this known blade, said longitudinal portions are connected through a spar and are separated from each other by two longitudinal slots, provided in the upper surface and the lower surface of said blade, respectively. The blade torsion is controlled by actuating means located at the bottom of the blade and carried by the mast of the rotary wing. When said actuating means control the blade torsion, the result is a relative sliding of said longitudinal portions along said slots.
This prior art US document seeks torsion stiffness negligible for said blade on purpose, which is achieved because the blade cross-section is dually opened by said longitudinal slots of upper and lower surfaces. The result is that the first torsion mode of said blade is very low, which makes monocyclic control of blade pitch likely to be difficult upon a forward flight. Such a blade can therefore only be operated in adaptive twist (quasi-static twist) and not in active (dynamic) twist.
Moreover, because the actuating means are concentrated at the bottom of the blade, the torsion force is located at this place so that the torsion angle caused by this force is (increasingly or decreasingly) monotonic along said blade.